Elderlypeoplearesometimessusceptibletoaconditionofchronicpainknownastrigeminal neuralgia. Peoplewiththisconditionexperiencespontaneousepisodesofsevere,oftenlancinatingpaininthedistributionofoneormorebranchesofthetrigeminalnerve.Frequentlythepainistriggeredbyweakmechanicalstimulationinthesameregion.Amajorcontributingfactortothispainfulstateappearstobemechanicaldamagetothetrigeminalganglionbyanarterythatimpingesontheganglion.Surgicaldisplacementofthearterycanoftenresolvethecondition.
The central pain pathways include the spinothalamic, spinoreticular, and spinomesencephalic tracts. The spinothalamic tract is the most important sensory pathway for somatic pain and thermal sensations from the body (see
Fig. 7.1A ). It also contributes to tactile sensation. The spinothalamic tract originates from second-order neurons located in the spinal cord (primarily laminae I and IV to VI). The axons of these cells cross to the opposite side of the cord at or near to their level of origin. They then ascend to the brain in the ventral part of the lateral funiculus and subsequently through the brainstem to the thalamus, where they terminate on third-order neurons (thalamocortical neurons), as described earlier. Spinothalamic cells conveying pain and temperature target the VPI portion of the ventroposterior complex (although some also end in the
Multireceptive spinothalamic VPL), the posterior nucleus, and the intralaminar nuclei tract cells of the thalamus. Nociceptive signals are then forwarded to 120 several cortical areas, including not only the somatosensory cortex but also cortical areas that are involved in affective responses, such as the cingulate gyrus and the insula, which have limbic system functions (see
Fig. 7.5
Most spinothalamic tract cells receive excitatory input from nociceptors in the skin, but many can also be excited by noxious stimulation of muscle, joints, or viscera. Few receive input only from viscera. Effective cutaneous stimuli include noxious mechanical, thermal (hot or cold), and chemical stimuli. Thus different spinothalamic tract cells respond in a manner appropriate for signaling noxious, thermal, or mechanical events. A
Some nociceptive spinothalamic tract cells receive convergent excitatory input from several different classes of cutaneous sensory receptors. For example, a given spino
Fig. 7.7A ). Such neurons are called wide–dynamic range cells or multirecep-120 tive cells because they are activated by stimuli with a wide range of intensities. Wide–dynamic range neurons signal mainly noxious events; weak responses to tactile stimuli appear to be ignored by the higher centers. However, in certain pathological conditions, these neurons may be suf ficiently activated by tactile stimuli to evoke a sensation of pain, possibly as a result of activity in sensitized afferents that were previously silent. This would explain some pain states in which activation of mechanoreceptors causes pain (mechanical allodynia). Other spinothalamic tract cells are activated only by noxious stimuli. Such neurons are often called high-threshold or nociceptive-specific cells (see
Fig. 7.7B
Because cells signaling visceral input also typically convey information from cutaneous receptors, the brain may misidentify the source of the pain. This phenomenon is called referred pain. A typical example is when the heart muscle becomes ischemic and pain is felt in the chest wall and left arm.